1. Field of the Invention
The present invention relates to a high brightness diamond light-emitting element using diamond as the light-emitting material.
2. Description of the Related Art
Diamond has a wide band gap of about 5.4 eV, and is capable of emitting a short wavelength light. Light-emitting elements using diamond have been studied as described below:
FIGS. 11 to 13 show prior art, electroluminescence (EL)-type light-emitting elements which are disclosed in Unexamined Japanese Patent Publication No. HEI 3-281594, wherein each of the light-emitting elements uses diamond as the light-emitting material. In FIG. 11, electrodes 52 are formed on the upper and lower surfaces of a diamond light-emitting layer 51. In FIG. 12, an electrode 52 is formed on the lower surface of a diamond light-emitting layer 51, an insulating layer 53 is formed on the upper surface of the light-emitting layer 51, and the electrode 52 is formed on the insulating layer 53.
In FIG. 13, insulating layers 53 are formed on both sides of a diamond light-emitting layer 51, and an electrodes 52 are formed on the insulating layers 53.
The above-described diamond light-emitting layers 51 are doped with an impurity such as N, B, Al, P, As, Sb, Ga or In to cause light emission.
In the light-emitting elements having such structures, the diamond layers 51 emit light when AC electric field is applied across the electrodes 52.
FIGS. 14 to 16 are cross sectional views of other prior art light-emitting elements, which are disclosed in Unexamined Japanese Patent Publication No. HEI 1-102893. FIG. 14 is a cross sectional view of a light-emitting element with an EL structure, where the light-emitting layer 54 is a semiconducting diamond doped with Al or B, and the entire surface of the semiconducting diamond light-emitting layer 54 is covered with an insulating layer 55. A metal electrode 56 is formed on the lower surface of the insulating layer 55, and a transparent electrode 57 is formed on the upper surface of the insulating layer 55. The semiconducting diamond layer 54 emits light when AC electric field is applied between the transparent electrode 57 and the metal electrode 56.
FIG. 15 is a cross sectional view of a light-emitting element having a metal electrode/semiconducting diamond (MS) structure. Here, a metal electrode 56 is formed in a specified area on the lower surface of a substrate 58. A semiconducting diamond light-emitting layer 54 is formed on the upper surface of the substrate 58. The other metal electrode 56 is formed in a specified area on the upper surface of the layer 54. The semiconducting diamond light-emitting layer 54 emits light when a forward current is allowed to flow between the metal electrodes 56. The light emission occurs by a mechanism that electrons are transported by way of donor levels and recombine with holes created by acceptors such as Al and B (carrier injection type).
FIG. 16 is a cross sectional view of a light-emitting element having a metal electrode/undoped intrinsic diamond/semiconducting diamond (MIS) structure. This is similar to the MS structure shown in FIG. 15, but an undoped intrinsic diamond layer 59 is sandwitched between the semiconducting diamond light-emitting layer 54 and the metal electrode 56.
In the MIS structure, the semiconducting diamond layer 54 emits light like the MS structure when a forward current is allowed to flow between the metal electrodes 56. The light emission occurs by the same mechanism as that described in the MS structure.
FIGS. 17 to 19 show cross sectional views of other prior art light-emitting elements, disclosed in Unexamined Japanese Patent Publication No. HEI 3-122093. In FIG. 17, a diamond light-emitting layer 61 is formed on a substrate 60, and a metal electrode 62 is selectively formed on the upper surface of the layer 61.
The light-emitting element shown in FIG. 18 is similar to that shown in FIG. 17, except that a low-resistance diamond layer 63 exists between the substrate 60 and the diamond layer 61.
The structure of the light-emitting element in FIG. 19 is different from that in FIG. 18 in that an insulating diamond layer 64 exists between the diamond layer 61 and the metal electrode 62.
The light-emitting elements shown in FIGS. 17 to 19 emit light when a forward current flows between the substrate 60 and the metal electrode 62. If the diamond layer 61 is doped with B, it emits a green light. On the other hand, if the diamond layer 61 is synthesized using a source gas which includes O.sub.2 to introduce lattice defects in the diamond layer, it emits a red light. When the diamond layer 61 is synthesized using a source gas which includes H.sub.2 O to introduce lattice defects, it emits a blue light.
FIG. 20 shows a cross sectional view of a light-emitting element disclosed in Unexamined Japanese Patent Publication No. HEI 3-222376. Here, a metal electrode 66 is selectively formed on the lower surface of a conductive substrate 65, and either p-type or n-type semiconducting diamond light-emitting element 67 is formed on the upper surface of the substrate 65. An undoped diamond film 68 is further formed on the upper surface of the semiconducting diamond layer 67, and finally a metal electrode 66 is selectively formed on the upper surface of the undoped diamond film 68.
When a forward voltage is applied between a pair of electrodes 66, the light-emitting element emits light because electrons recombine with holes by way of the defect levels in the semiconducting diamond layer 67.
The above-described prior art light-emitting elements have the following problems: the EL elements shown in FIGS. 11 to 14 require an operation voltage as high as 300 V, which is unsuitable for practical use.
In the light-emitting elements shown in FIGS. 15 and 16, the light emission depends on the density of the donor levels in the diamond layer 54 due to impurities such as N. However, the density of such doner levels is very low, and thus the brightness is limited.
For the light-emitting elements shown in FIGS. 17 to 20, the defect density is also limited and the brightness is low.